The February 15, 1939 issue of the Physical Review included a letter by Niels Bohr entitled “Resonance in Uranium and Thorium Disintegration and the Phenomenon of Nuclear Fission.”1 Dated February 7, the letter is about a page and a half in length, contains no equations, and was sent from the Institute for Advanced Study in Princeton. It contains a very significant observation about nuclear fission. Although he says nothing about this in his letter, Bohr was aware of its implications for the development of atomic weapons.

A few months earlier, the physicist Otto Frisch had spent his Christmas vacation with his aunt, Lise Meitner, in Sweden, where she had fled from Germany in 1938. In Berlin, Meitner had been associated with the radio chemists Fritz Strassmann and Otto Hahn. They had been studying the reaction of uranium when bombarded by neutrons, research that had begun with Enrico Fermi and his group in Rome. Like Fermi, Hahn and Strassman had expected to find evidence for the creation of new transuranic elements. They found barium instead, an element with about half the mass of uranium. While walking together in the woods outside of Kungälv, Meitner and Frisch realized that what must have happened was the fission of the uranium nucleus.

When Frisch returned to Copenhagen, Bohr was preparing to leave for Princeton. Bohr immediately grasped the significance of Frisch’s conversation with Meitner. He agreed to say nothing until they had published their paper. Unfortunately, Bohr forgot to tell his assistant, Léon Rosenfeld, about the arrangement. After Bohr and Rosenfeld reached New York, Rosenfeld went ahead to Princeton where he told everyone. All the physicists in the United States who worked on such things heard the news within a matter of days. Upon arriving in Princeton himself, Bohr began working with John Wheeler. The two men began working on what was to become their monumental study of fission.

On February 5, 1939, Bohr had breakfast with Rosenfeld at the Princeton faculty club. The pair were joined by George Placzek, a Czech physicist who had worked with Bohr in Copenhagen. Placzek was known for his skepticism. He told Bohr that his physical model of fission must be nonsense. Placzek’s argument was straightforward. If the energy of the impinging neutrons was reduced to about twenty-five electron volts, the reaction rate for neutron capture would undergo a sharp rise, or resonance. No such resonance had been observed for fission. Both processes appealed to the same nuclear model. Bohr left the room in a state of agitation, followed by Rosenfeld, who knew better than to speak to him at such moments. Shortly after returning to his office, which had been loaned to him by Albert Einstein, Bohr announced that he understood everything. What he understood became the subject of the letter that appeared in the Physical Review.

Uranium ore is comprised of more than ninety-nine percent 238U. The remainder is primarily 235U. Suppose that it was the 238U that captured the neutrons, and it was only the 235U that had undergone fission. This would explain why in certain experiments a resonance peak was observed and in others not. Bohr was able to explain this. The heavy nuclei could be modeled as liquid drops. If an impinging neutron agitated a drop, with sufficient ambient energy, it could cause the drop to deform and break up. Bohr understood that there was a difference between 238U and 235U. Upon absorbing a neutron, 238U becomes 239U, while 235U becomes 236U. The latter is termed an even–even nucleus due to its even number of neutrons and protons. These nuclei are more tightly bound than an even–odd nucleus, such as 239U. The larger binding energy is manifested in a higher energy release when the neutron is absorbed. This, in turn, distorts the liquid drop. Nuclei such as 235U are fissile. This is clearly spelled out in the penultimate paragraph of Bohr’s letter. What is not mentioned are his thoughts on the subject of nuclear weapons. Much to his relief, Bohr had come to the conclusion that they were impossible to produce. Separating sufficient quantities of the isotopes would, he later remarked, require the resources of an entire country.

While reviewing the letter, I noticed a couple of oddities. Bohr twice refers to Otto Frisch as “R. Frisch” in the footnotes. Frisch’s middle name was Robert and his papers were signed with the initials “O. R.” Bohr, it seems, left off the first initial. Placzek is not mentioned anywhere in the letter despite the fact that he asked the key question. This should have been acknowledged. Incidentally, Wheeler famously bet Placzek $18.36 to $0.01—the ratio of the mass of the proton to the electron—that 235U rather than 238U was responsible for slow neutron fission.3 When Wheeler won the bet, Placzek sent him a money order for one cent.

Thorium enters into the picture because it has an isotope that occurs in more than trace amounts. This is 232Th. If it captures a neutron it turns into 233Th, which in a sequence of decays turns into 233U. This isotope is fissile in the same way as 235U. &larrhk;

John Wheeler, “Fission in 1939: The Puzzle and the Promise,” Annual Review of Nuclear and Particle Science 39 (1989): xiii–xxviii. &larrhk;